In a 4-stroke cycle gasoline engine, the combustion process is, by design, initiated by the spark-plug at the right crank angle, leading to optimum energy output. If the fuel-air mixture ignites under compression, either prior to the spark or in the unburned fuel-air mixture being heated and compressed by the propagating flame, abnormal combustion may occur. Examples of this are engine knock (detonation after the spark) or pre-ignition. These undesirable events may result in engine damage.
The resistance to abnormal combustion events of a fuel is rated on one of several octane scales, such as the Research Octane Number (RON), Motor Octane Number (MON), or the Supercharged Rich Octane method. Higher octane numbers indicate a resistance to combustion, and are associated with in increased ignition delay. Generally, aromatics, naphthenes and branched paraffinic molecules increase the octane number of a fuel, while linear paraffins decrease the octane number of a fuel. An example of this is the addition of diesel fuel (which consists of long, linear hydrocarbons) to gasoline causes a decrease in the octane number of the gasoline.
Oxygenate additives such as methanol, ethanol, and MTBE are known to increase octane number. However, there are performance concerns associated with methanol (e.g., corrosion) and ethanol (e.g., elastomer compatibility), and environmental concerns associated with MTBE. In addition, these oxygenates are not suitable for use in a lubricant composition.
Today's high performance engines are trending toward higher compression ratios, in order to generate higher power at a given engine displacement. As the compression ratio increases, the fuel-air mixture has a higher propensity to ignite by compression, resulting in detonation of the unburned end gases (knocking) or pre-ignition.
Traditional spark knocking can be controlled by retarding spark timing or by reducing the super- or turbo-charger boost pressure. Hot-spot pre-ignition is prevented by engine hardware design and limiting the temperatures in the combustion chamber. However, these measures also reduce the efficiency of the engine. An approach preferred by engine manufacturers is to use fuels that are less likely to be ignited by compression.
Engine oils usually contain 80-90% of hydrocarbon base oils. These hydrocarbons are similar to diesel fuel in chemical structure and ignite easily under compression. During normal engine operation, some of the engine oil exists in the combustion chamber, leading to the concern that engine oil contributes to engine knocking and pre-ignition.
Under high brake mean effective pressure (BMEP) and low engine speed (RPM), some modern internal engines experience an abnormal combustion phenomenon called low speed pre-ignition (LSPI) or “super knock”. It is known that LSPI can lead to severe engine damage.
Although engine knocking and pre-ignition problems can be and are being resolved by optimization of internal engine components and by the use of new component technology such as electronic controls, modification of the lubricating oil compositions used to lubricate such engines and fuel compositions would be desirable. For example, it would be desirable to develop new lubricating oil formulations or fuel compositions which are particularly useful in internal combustion engines and, when used in internal combustion engines, will prevent or minimize the engine knocking and pre-ignition problems. It is desired that the lubricating oil composition and fuel composition be useful in lubricating gasoline-fueled, diesel-fueled, and natural gas, liquefied petroleum gas, or dimethyl ether-fueled spark ignition engines.